"I don't see anything particularly wrong with a muttered comment to a neighbour in this situation."

Although I do think you should sit and listen to the performer, in most circumstances you could expect it would not be sufficient to distract either the performer or other members of the audience. It may not be particularly wrong, although that doesn't make it right, but usually you could expect to get away with it without upsetting anyone.

On this particular occasion, however, it was sufficient to distract the performer. In these circumstances I think the performer should be given the benefit of the doubt. The impression Pierre has given, at least to me, is that he thinks the performer was in the wrong for allowing himself to be distracted, and that he (Pierre) was in no way at fault.

It does make a difference if the performer is a floor singer rather than a pro. A floor singer is more likely to be susceptible to distractions and disturbances, and perhaps less adept at responding to them. The audience should be sympathetic to this and cut them more slack.

Dick Gaughan, a supreme professional, has said that one of the things which distinguishes him as a pro from even the best amateurs is that at some time during his career everything that could go wrong has gone wrong, and he's learned how to deal with it.

I agree the performer's response was a bit peremptory and I can understand Pierre being upset and annoyed by it. But I still believe it was the performer's prerogative to ask him to be quiet.

To answer the OP's original question, I believe that in a concert the performer (and the audience for that matter) is entitled to silence and attention from the audience. If he doesn't get this, to the extent that it is affecting his performance and/or the enjoyment of the rest of the audience, then he his entitled to express his feelings. Ideally, he should do this in a tactful and humorous manner which does not upset anyone, but this is not always possible.